1. Field of the Invention
This invention relates to a method for formation of a deposited film, which is useful for the preparation of a functional film, particularly a semiconductive deposited film being useful for uses such as semiconductor device, electronic device such as optical input sensor device for optical image inputting device, photosensitive device for electrophotography, etc.
2. Description of the Related Art
Hitherto, for functional films, particularly crystalline semiconductor films, individually suitable film forming methods have been employed from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of silicon deposited films such as amorphous or polycrystalline non-single crystalline silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X), etc., (hereinafter abbreviated as "NON-Si (H,X)", particularly "A--Si (H,X)" when indicating an amorphous silicon and "poly--Si (H,X)" when indicating a polycrystalline silicon) (the so called microcrystalline silicon is included within the category of A--Si (H,X) as a matter of course), there have been attempted the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been used as the optimum method and industrialized.
However, the reaction process in formation of a silicon deposited film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and its reaction mechanism involves many ambiguous points. Also, there are a large number of parameters for formation of a deposited film such as substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc. On a count of the use of a combination of such a large number of parameters, the plasma may sometimes become unstable state, whereby marked deleterious influences were exerted frequently on the deposited film formed. Besides, the parameters characteristic of the device must be selected for each device and therefore it has been difficult to generalize the production conditions under the present situation.
Also, in the case of the plasma CVD method, since plasma is generated by high frequency or microwave, etc., in the film forming space in which a substrate on which film is formed is arranged, electrons or a number of ion species generated thereby may give damages to the film in the film forming process to cause lowering in film quality or unevenness of film quality. Moreover, the conditions for crystallization of deposited film are narrow and therefore it has been deemed to be difficult to produce polycrystalline deposited films having stable characteristics.
Whereas, for formation of an epitaxial deposited film such as of silicon, germanium, the group II-VI and the group III-V semiconductors, etc., there have been used the gas phase epitaxy and the liquid phase epitaxy as classified broadly (generally speaking, the strict definition of epitaxy is to grow another single crystal on a single crystal, both having the same single crystal axes, but here epitaxy is interpreted in a broader sense and it is not limited to the growth onto a single crystal substrate).
The liquid phase epitaxy is a method for the deposition of a semiconductor crystal on a substrate by dissolving a starting material for semiconductor at an elevated temperature to a supersaturated state in a solvent of a metal which is molten to a liquid and cooling the solution. According to this method, since the crystals are prepared under the state most approximate to thermal equilibrium among various epitaxy techniques, crystals having high perfectness can be obtained, but on the other hand, mass productivity is poor and also the surface state is bad. For such reasons, in an optical device which requires an epitaxial layer which is thin and also uniform in thickness, problems are accompanied such as yield in device production, or influences exerted on the device characteristics, etc., and therefore this method is not frequently used.
On the other hand, the gas phase epitaxy has been attempted by physical methods such as the vacuum vapor deposition method, the sputtering method, etc., or chemical methods such as hydrogen reduction of a metal chloride, or thermal pyrolysis of an organic metal or a metal hydride. Among them, the molecular ray epitaxy which is a kind of the vacuum vapor deposition method is a dry process under ultra-high vacuum, and therefore high purification and low temperature growth of crystals are possible, whereby there is the advantage that composition and concentration can be well controlled to give a relatively even deposited film. However, in addition to an enormous cost required for the film forming device, the surface defect density is great, and no effective method for controlling directionality of molecular ray has been developed, and also enlargement of area is difficult and mass productivity is not so good. Due to such many problems, it has not been industrialized yet.
The hydrogen reduction method of a metal chloride or the thermal pyrolysis method of an organic metal or a metal hydride are generally called the halide CVD method, the hydride CVD method, the MO--CVD method. For these methods, since the film forming apparatus can be manufactured with relative ease and also metal chloride, metal hydrides, and organic metals being the starting materials, those with high purities are now readily available, they have been studied widely at the present time and application for various devices has been investigated.
However, in these methods, it is required to heat the substrate to at least an elevated temperature at which the reduction reaction or thermal pyrolysis reaction can occur and therefore the scope of choice of the substrate material is limited, and also contamination with impurities such as carbon or halogen, etc., is liable to be caused if decomposition of the starting material is insufficient, thus having the drawback that controllability of doping is poor. Also, depending on the application use of the deposited film, it is desired to effect mass production having reproducibility with full satisfaction of enlarged area, uniformization of film thickness as well as uniformness of film quality and yet at a high speed film formation, under the present invention. However, no technique which enables mass production while maintaining practical characteristics satisfying such demands has been established yet.